So-called concrete tilt-up building panels or slabs are increasingly used in the construction industry, primarily for commercial and industrial buildings. In such constructions concrete panels are formed on the floor of the building or on an adjoining level ground surface by pouring the panels in their horizontal position. After the concrete has hardened, the panels are lifted into a vertical or upright position and interconnected to form the walls of a building.
Due to their large weight special provisions must be made to pick up the panels with a crane or the like without damaging the panels, without permitting the panels to swing freely which would endanger life and property, and, most importantly, without permitting any of the panels to drop during the erecting process. In the past a variety of ways and means for accomplishing this task have been proposed or attempted. The following U.S. Pat. Nos. are exemplary of such attempts: 2,794,336; 3,431,012; 3,456,547; 3,652,118; and 3,705,469.
A common characteristic of these patents is that each of them discloses the idea of embedding in the concrete panel or slab a suitably formed insert that defines an aperture or opening into which a mating pickup device can be inserted as a preliminary to a panel-hoisting operation. In one prior art attempt, the insert defines an interiorly threaded surface, such as is disclosed in U.S. Pat. Nos. 3,456,547 or 2,794,336, into which a correspondingly threaded shaft of the pickup device can be inserted. In other instances, as shown in U.S. Pat. Nos. 3,431,012 or 3,705,469 for example, crossbars or the like traverse the openings in the panel and the pickup device has correspondingly shaped members for engaging the crossbars and thereby locking the device to the insert.
Prior art concrete panel pickup systems (which include the concrete embedded insert and the pickup device that is removably attached to the insert) have various relative advantages and disadvantages. There are some which have proved unreliable in the field and at times they have resulted in panels dropping from the crane. Such events, of course, can be disastrous if the dropping panel strikes a person; in addition, the panel is normally cracked and lost. Other prior art pickup systems have proved relatively safe. However, in operation they proved to be less than fully satisfactory.
For example, panel pickup systems employing threaded inserts normally have the insert in the form of a tightly wound spring into which a threaded shaft must be inserted. If the shaft is threaded into the insert, the task is time-consuming and adds to the overall cost of erecting a building wall. If the shaft is a split shaft (such as is disclosed in U.S. Pat. No. 3,456,547) in which two shaft halves are collapsed, then axially inserted into the insert, and thereafter spread apart with a wedge, even slight foreign matter on the interior insert surface, such, for example, as hardened concrete particles, can prevent proper operation of the device.
If this is the case a workman must first clean the rather inaccessible insert interior before the insert can be applied. In addition to requiring costly labor to clean the insert and mount the pickup device, auxiliary equipment such as the pickup crane as well as personnel such as the crane operator and assisting workmen remains idle.
Another serious shortcoming of prior art panel pickup systems is the manner in which the pickup devices are removed from the panel-embedded inserts. Frequently, it is necessary for a workman to place a ladder against the upright panel, climb the ladder to the height of the pickup devices, and then release the devices, manually or with the help of such tools as hammers, screwdrivers, pliers and the like. Parts that can become wedged make this task even more difficult.
Once the pickup device is released there are normally one or more loose parts which must be saved for the application of the pickup device to the next insert. This alone is tedious. Moreover, the loose parts can drop and injure bystanders and become damaged. In the rough surroundings of general construction sites such loose parts are dangerous to persons and the parts themselves are in constant danger of being lost or damaged hence requiring replacement.
Since the discussed difficulties of attaching the pickup device are rather common with prior art systems and since the erection of large buildings may require erection of hundreds of tilt-up panels, the cost added by unsatisfactorily functioning pickup systems can be substantial.